Polyethylene resins are used in many applications requiring good physical properties, easy processing, sufficient melt strength for formation of films or blow molded articles. Linear Low Density Polyethylene (LLDPE) has very desirable physical properties but mostly lack sufficient melt strength to be used in certain film applications such as large bubble films or in blow molding applications, pipe applications and extrusion coating applications. Similarly High Density Polyethylene (HDPE) suffers from the same shortcoming as LLDPE with respect to melt strength. Low Density Polyethylene (LDPE) made by the high pressure free radical process while having significantly higher melt strength suffers from lack of good mechanical properties. In many applications a blend of LLDPE or HDPE with LDPE is used. Although those blends do have higher melt strength, the addition of even small amounts of LDPE, such as 10 to 20% LDPE, does cause a significant drop in the mechanical properties such as tear and dart impact resistance. Even in the case of LDPE there are areas a higher melt strength could be very beneficial such as extrusion coating or collation shrink.
There is a great need to be able to impart a required melt strength increase in an economical and controlled manner, with small cost increase. Such an improvement will enhance the use of polyethylene resins across the applications.
Polyethylene has desirable properties that have helped to make it the highest volume polymer manufactured. Polyethylene can be made in different processes in order to give different properties. Known families of polyethylene include high density polyethylene (HDPE), linear low density polyethylene (LLDPE), and low density polyethylene made using high pressure reactors (LDPE). Within these broad classes many variations exist resulting from different types of polyolefin process technologies (for example, solution, slurry or gas phase) or from the use of different catalysts (for example, Ziegler-Natta or constrained geometry catalysts). The desired application requires a careful balance of rheological properties which will lead a person of skill in the art to select one type of polyethylene over another. In many applications, such as blow-molding and blown film applications, melt strength of the polyethylene is a key parameter.
The melt strength is a practical measurement that can predict material performance in elongation. In melt processing, good melt strength is important to maintain stability during processes such as coating, blown film production, fiber spinning and foamed parts. Melt strength is related to several processing parameters such as bubble stability and therefore thickness variation during blown film production; parison formation during blow molding; sagging during profile extrusion; cell formation during foaming; more stable thickness distribution during sheet/film thermoforming.
This property can be enhanced by using resins with higher molecular weight, but such resins will generally require more robust equipment and more energy use because they tend to generate higher extrusion pressure during the extrusion process. Therefore, properties must be balanced to provide an acceptable combination of physical properties and processability.
In thick film applications, blends of LDPE and LLDPE are used in order to obtain a balance of processability (extruder amps and pressure) and film mechanical properties. In this blend the LDPE component is the processability component whereas the LLDPE is the mechanical component. Therefore, the ability to decrease the LDPE portion of the blend should increase the mechanical properties of the blend. Through this invention, the ability to increase the melt strength of the LLDPE component allows the use of a higher percentage of LLDPE in the blend, thus increasing the mechanical properties without sacrificing processability or the creation of unacceptable levels of insoluble material.
The present invention is a new method for increasing the melt strength of polyethylene involving reacting molten polyethylene with specified free radical generator with a specific peak decomposition temperature and decomposition energy through regular extrusion processing. Accordingly, one aspect of the invention is a method for increasing the melt strength of a polyethylene resin comprising first selecting a polyethylene resin having a density, as determined according to ASTM D792, in the range of from 0.90 g/cm3 to 0.970 g/cm3, and a melt index, as determined according to ASTM D1238 (2.16 kg, 190° C.), in the range of from 0.01 g/10 min to 30 g/10 min and then reacting the specified free radical generator with the polyethylene resin in an amount and under conditions sufficient to increase the melt strength of the polyethylene resin.
The present invention is a new process for increasing the melt strength of polyethylene involving reacting molten polyethylene with the specified free radical generator through regular extrusion processing. Accordingly, one aspect of the invention is a method for increasing the melt strength of a polyethylene resin comprising first selecting a polyethylene resin having a density, as determined according to ASTM D792, in the range of from 0.900 g/cm3 to 0.970 g/cm3, and a melt index, as determined according to ASTM D1238 (2.16 kg, 190° C.), in the range of from 0.01 g/10 min to 30 g/10 min and then reacting the specified free radical generator with the polyethylene resin in an amount and under conditions sufficient to increase the melt strength of the polyethylene resin.
The present invention is a new process that increases the melt strength of the resin.
The present invention is a new process that increases the Viscosity Ratio of the resin, indicating good processability.